How to interpret ankle-brachial index (ABI) waveforms

During an automated ankle-brachial index (ABI) test, ABI ratios are calculated and included in the patient report. While waveforms are only audible when using Doppler during a manual ABI, an automated ABI machine’s Doppler probe also provides analog waveforms, which form an important component of the report.

Understanding how to interpret both the audible and analog waveforms of an automated ABI machine is especially important in circumstances where the ABI ratios are inaccurate and irrelevant. For example, some vessels are so calcified that they resist compression and falsely elevate the ABI ratio.

Before we get into how to interpret audible and analog waveforms, let’s review how to position the Doppler probe when performing an ABI. Pulse sounds (e.g., audible waveforms) and analog waveforms are qualitative data produced by the Doppler probe. The sound is crucial, but it is not recorded. Only the analog waveforms are recorded, and their quality is dependent on the skill of the person using the Doppler probe.

It is easy to make a healthy vessel look diseased, but it is not easy to make a diseased vessel look healthy. In other words, audible and analog waveforms may overestimate the presence of disease if the technique is poor.

Good technique comes with practice and experience. Ideally, the pen is held at 45° (or less) to the vessel or skin, and is pointed towards the heart. The more perpendicular you are to the vessel, the more you will overestimate the severity of disease.

Artifacts can appear on analog waveforms as multiple tiny peaks and pits. Artifacts are errors, not true peaks and pits, and they can make waveforms difficult to classify.

There are two reasons why artifacts may be introduced:

1. The user has a shaky hand.
2. There is venous flow interference.

A tip to avoid venous flow interference is to press gently on the vein with the probe. Since the vein has lower pressure, it will collapse. The venous flow will pause and allow for a clear arterial Doppler reading without artifacts.

In general, waveforms that consist of two peaks and one pit are ideal and considered normal. These are called triphasic waveforms.

As peripheral arterial disease (PAD) worsens from mild to severe, the waveforms change. First, they decrease in the number of peaks and pits, then they develop rounded upstrokes or humpback peaks (e.g., wide peaks without pits), and finally, all peaks and pits become absent.

There are five stages of waveform degradation that occur with worsening PAD:

1. Biphasic (mild)
2. Weak biphasic (moderate)
3. High monophasic (moderate)
4. Dampened monophasic (severe)
5. Absent (critical)

First, let’s cover what triphasic waveforms look and sound like. Then, we will describe each stage of waveform degradation so you can master how to interpret both audible and analog waveforms across PAD stages.

Triphasic waveforms (normal)

The healthiest waveforms are called triphasic, meaning they have two peaks and one pit that are both audible and visible. A triphasic waveform indicates that a shift of direction in blood flow is occurring, which reflects normal vessel flexibility.

A triphasic waveform features a sharp incline to the tallest peak; the upstroke represents an acceleration of blood flow to a peak systole. The pit is early diastole, which is the reversal of flow. The second peak is late diastole, which is the forward flow. Essentially, triphasic waveforms represent blood flow that is normal at rest.

With audible triphasic waveforms, you’ll notice three distinct sounds that follow the pattern peak-pit-peak (e.g., one pit and two peaks).

Biphasic waveforms (mild)

Artery walls lose elasticity as they become diseased, which is reflected in a loss of humps in the ABI waveforms. Audible biphasic waveforms only have two sounds, and visually (e.g., on analog) have only one peak and one pit. Notably, biphasic waveforms still have a sharp upstroke to the tallest peak.

The loss of the second peak can sometimes be due to an asymptomatic loss of elasticity in the artery. This is due to wall calcification, which is worse with atherosclerosis and diabetes but can also develop naturally with age. Wall calcification can become severe at a younger age in patients with diabetes.

Patients with biphasic waveforms usually present with symptoms of intermittent claudication. However, biphasic waveforms can also be found in patients with mild to moderate arterial insufficiency. Correlating the waveforms with the ABI ratios helps decipher the cause of the biphasic waveforms. For example, if the ABI is 1.0 and the waveform is biphasic, there could be mild calcification in the vessels but no significant atherosclerosis. Biphasic waveforms obtained with an ABI of 0.7 indicate mild to moderate PAD.

When listening to biphasic waveforms, you’ll notice only two distinct sounds. As long as the waveforms are multiphasic, biphasic, or triphasic (e.g., not monophasic), the situation is not an immediate surgical concern.

Weak biphasic waveforms (moderate)

Waveforms with a sharp upstroke and shallow pits are called weak biphasic. They are found when there is moderate arterial insufficiency.

Weak biphasic waveforms are best analyzed using analog (not audible) waveforms. Similar to the biphasic waveforms, you can still expect to hear one peak and one pit. But, the difference with weak biphasic waveforms is that the upstroke (e.g., peak) is usually louder than the pit.

High monophasic waveforms (moderate)

When the waveforms have no pits but still have one peak with a sharp upstroke, they are called high monophasic. These waveforms also represent moderate arterial insufficiency.

Distinguishing between high monophasic and dampened monophasic waveforms can be difficult. It involves a subjective component which is dependent on the experience of the operator. Experienced operators can appreciate the sharp upstroke found with high monophasic—as opposed to the dampened waveforms that are seen with more severe forms of PAD.

Dampened monophasic waveforms (severe)

Dampened humpback monophasic waveforms indicate severe PAD. There is a slow-rising peak above the baseline and the upstroke is rounded—not sharp. Dampened monophasic waveforms can be seen with non-healing ischemic ulcers.

When listening to dampened monophasic waveforms, you’ll hear only one peak and continuous flow throughout the cardiac cycles.

Absent waveforms (critical)

Absent ABI waveforms are due to a lack of peripheral pulses. They are found in patients with rest pain and critical limb ischemia.